1. Field of the Invention
The present invention relates to a process for preparing propylene oxide by reacting propylene and a peroxide compound, in the presence of a pretreated titanium silicalite epoxidation catalyst.
2. Description of Background and Related Art
In the preparation of propylene oxide, the customary processes in the prior art involve reacting propylene with hydrogen peroxide to produce propylene oxide. The process is usually carried out in one or more stages. For example, U.S. Pat. Nos. 6,479,680; 6,849,162; 6,867,312; 6,881,853; 6,884,898; 6,960,671; 7,173,143; 7,332,634; 7,378,536; and 7,449,590; all of which are incorporated herein by reference, disclose a process for the preparation of propylene oxide using hydrogen peroxide in the presence of a catalyst such as a titanium silicalite catalyst, and in the presence of a solvent such as methanol.
A problem with the prior art propylene oxide production processes relates to the use of methanol as a solvent for the processes. While methanol is a necessary reaction component of the peroxide reaction to obtain high activity, the methanol is generally used in large excesses (for example, from 50-90 weight percent) to ensure that the reaction mixture remains as one liquid phase. For example, Clerici et al., “Epoxidation of Lower Olefins with Hydrogen Peroxide and Titanium Silicalite,” Journal of Catalysis, 1993, 140, 71-83, discloses a process that is representative of titanium silicalite/hydrogen peroxide epoxidation processes, wherein the methanol level used in one example is approximately 97%. The use of an excessive amount of methanol, as described in the prior art processes, results in the formation of a single phase during the reaction; and the prior art processes suffer from the formation of byproducts, for example, byproducts formed from the reaction of methanol and water, wherein such byproducts are solubilized in the organic phase by methanol, with propylene oxide. The use of large quantities of methanol also results in the need for large towers for propylene oxide production facilities, and for a high energy consumption for the purification of the product produced on a commercial scale.
The problems of the prior art processes may be solved by reducing the methanol concentration or removing methanol entirely from the reaction mixture. However, in the epoxidation of propylene, reducing or eliminating methanol concentration in the known processes creates a reaction system with two liquid phases, which results in lower propylene oxide yield, lower hydrogen peroxide (H2O2) selectivity to propylene oxide, and/or longer reaction times.
Zhang et al., “Effects of Organic Solvent Addition on the Epoxidation of Propene Catalyzed by TS-1,” Reaction Kinetics and Catalysis Letters, 2007, 92(1), 49-54, discusses the use of solvent mixtures with methanol for the epoxidation of propylene. Zhang et al. disclose that replacing approximately 24% of the methanol in Zhang et al.'s system with other solvents such as CCl4, toluene, or 1, 2-dichloroethane, results in increased selectivity and less clogging of the catalyst pores, as measured by thermogravimetric analysis (TGA) and pore volume analyses. The mixture of methanol and 1, 2-dichloroethane, of Zhang et al.'s system, which gives a total methanol composition of 60%, inhibits the decomposition of H2O2; and inhibits the reaction of propylene oxide to form propylene glycol and propylene glycol mono-methyl ethers, while retaining the H2O2 conversion as compared to the use of methanol only. However, although Zhang et al. describe using mixtures of solvents to increase the selectivity of the reaction, the total solvent amounts used create single phase reaction conditions. Thus, prior art processes which use solvent mixtures still use a high level of methanol and/or a single liquid phase.
In summary, the disadvantages of the known processes described in the above prior art include the following:
(1) The prior art processes use high levels of methanol; and thus, the high levels of methanol must be separated from the product and recycled. This creates high energy usage for the process and associated high costs.
(2) The prior art processes use a first separation step wherein solvent and unreacted reactant are recovered and recycled. This requires the use of a high temperature in the bottoms of distillations towers, which in turn requires the use of high temperatures throughout the distillation towers. Any water in the feed to the distillation towers remains in contact with the product in the distillation towers; and thus the available water provides an opportunity to react with the product to form undesired by-products.
It is therefore desired to provide a process that does not have the problems of the above prior art processes and that can be operated at reaction conditions which address all of the above issues and maintain a high catalyst activity